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Washington State News

Noxious Weed Threatens Washington Horses

(The Center Square) — A noxious weed native to Europe and Asia has invaded Washington’s pasturelands, sickening at least 25 horses.

Washington State Department of Agriculture warns livestock owners to check hay supplies for hoary alyssum, which may cause fever, diarrhea, edema, laminitis or death if consumed in larger quantities.

Hoary alyssum has been detected in Lewis and Jefferson counties, according to social media reports.

“I am in Jefferson county, it has definitely invaded my very sandy pastures,” Ali McMahon wrote on Facebook, adding that she had not seen any wildlife consuming the plant.

“I have a friend that got some alfalfa that had some in it and almost killed 20 of their horses,” Sarah Huges of Onalaska wrote.

Hoary alyssum grows to from 1 to 3.5 feet in height, with stems covered in hair-like fibers giving it a silver-gray color and white flowers. The weed thrives in dry conditions such as sandy or gravelly soil. Overgrazed pastures, stressed meadows and roadsides are liable to infestation.

Well-maintained pastures generally suppress the week through competition according to the Washington State Noxious Weed Control Board. Small infestations can be dug out by hand. Mowing also prevents the plant from reproducing.

WSDA data from 2018 shows previous infestation of greater than 1,000 acres in Okanogan, Ferry and Stevens counties; infestation of over 100 acres in Spokane and Klikitat counties; more than 10 acres in Pend Oreille county; and fewer than 10 acres in Clallam, Chelan, Kittitas, Clark and Lincoln counties. The weed was also detected in Pierce County.

The Weed Control Board warns landowners not to produce hay from fields containing hoary alyssum and to check their county noxious weed coordinator for information on using herbicides.

WSDA asks anyone discovering hoary alyssum on their property or in hay supplies to report it to or (360) 902-1844.